1. Field of the Invention
The present invention relates to ceramic-NiO composite powders which can be used to form ceramic-NiO composite body anodes. These anodes possess an interpenetrating network structure and can be used in solid oxide fuel cell (SOFC) by The present invention also describes methods of preparing these powder, anodes and fuel cells.
2. Background of the Related Art
A Fuel cell is a battery which can convert chemical energy generated by fuel oxidation into electrical energy. There are two major types of fuel cells: a low temperature type and a high temperature type. Since the hydrogen-oxygen fuel cell was introduced in the 1950s, the fuel cell has been continuously improved. Two of the most important fuel cell improvements involved high-temperature types: the molten carbonate fuel cell (MCFC) and the solid oxide fuel cell (SOFC hereinafter).
SOFC uses electrolytes having oxygen or hydrogen ion conductivity and is operated at among the highest temperature for fuel cells of its kind, about 700 to 1,000° C. Since all of its constituents are made of solid materials, it provides a rather simple structure compared to other types of fuel cells and also does not have problems connected with corrosion or loss/replenishment of electrolytes. In addition, SOFCs do not require the use of expensive nickel as a material but can directly use conventional hydrocarbon fuel without the use of an additional modifier. Furthermore, it enables combined heat generation using waste heat because it releases high temperature gases.
SOFCs in general consist of oxygen ion conductive electrolytes as well as cathodes and anodes which are located on opposite ends of the SOFC. The principle of operation of the SOFC is as follows. The oxygen ions generated at the cathode by the reduction of oxygen move through the electrolytes to the anode and react with hydrogen provided nearby, thus producing water. Electrons are generated at the anode while they are consumed at the cathode. Connection of the two electrodes generates electricity.
Anodes of SOFCs is usually made of nickel and ceramic. In a conventional method, nickel oxide (“NiO”) powder and yttria-stabilized zirconia (“YSZ”) powder were mixed, formed, sintered to form a sintered body and then heat treated under reduction atmosphere to produce Ni-YSZ cermet (KR 10-344936).
The above method is advantageous in that its process is simple. However, the interactions between YSZ powder and YSZ, NiO powder and NiO powder or YSZ powder and NiO, respectively, are different from each other, and thus even under the same conditions the two powders do not distribute concomitantly but usually coagulate together. When there is a particularly large difference in size between the two particles, isoagglutination occurs which results in a nonuniform microstructure at the anode.
The lack of powder uniformity influences the electrical conductivity, gas permeability, three-phased boundary reactivity, and other physical properties of the anode. The non-uniform microstructure of the anode also introduces procedural defects in the YSZ solid electrolyte layer and interfacial strength thereby deteriorating durability, mechanical strength and output characteristics of the manufactured unit cell battery.
More specifically, the lack of uniform crystal grain and pore size leads to denseness and coarseness in the Ni phase. This coarseness causes a change in volume during the heat cycle and redox reaction which subsequently leads to electrolyte damage. Further, the decrease in the size of the Ni, YSZ and pore three-phased boundary leads to a decrease in electrochemical activity and an increase in activation polarization resistance. This subsequently lowers the resulting output of the unit cell battery.